Wave Cloud Animation

March 31, 2009


Photographer: Don Collins, Warren Wilson College
Summary Author: Don Collins

The Earth Science Picture of the Day for March 27 shows wave clouds above Sydney, Australia. Today we’re featuring a time-lapse animation of wave clouds (altocumulus undulates) as photographed near Asheville, North Carolina on January 17, 2009. These wave clouds form downstream of elevated terrain and result from a combination of wind shear (different wind velocities at different altitudes) and heat released from condensing water vapor. When winds moderately increase with height above a mountain barrier, the oscillation caused by the mountains sets up a train of lee waves. These waves are referred to as stationary gravity waves. As the air climbs the crest of a wave, adiabatic expansion due to lower pressure cools and condenses water vapor, making the wave visible. If the air is highly stable, the clouded air then sinks back to the lower level where the vapor evaporates -- and the cycle repeats. The air on this midwinter’s day was very cold. An approaching warm front was responsible for the moisture input. Careful inspection of the animation shows sharp wavefronts with breaking and sinking evaporating clouds on the lee sides of each wave.

Photo details: Photos were taken every 10 seconds. The cloud array was receding, so judicious cropping and alignment had to be invoked to catch-up with the receding waves. The animation is played back at 10 frames/sec – a speed-up factor of 100.